Liquid water absorption and scattering effects in DOAS retrievals over oceans
Spectral effects of liquid water are present in absorption (differential optical absorption spectroscopy – DOAS) measurements above the ocean and, if insufficiently removed, may interfere with trace gas absorptions, leading to wrong results. Currently available literature cross sections of liquid wa...
| Main Authors: | , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2014-12-01
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| Series: | Atmospheric Measurement Techniques |
| Online Access: | http://www.atmos-meas-tech.net/7/4203/2014/amt-7-4203-2014.pdf |
| _version_ | 1811229758517673984 |
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| author | E. Peters F. Wittrock A. Richter L. M. A. Alvarado V. V. Rozanov J. P. Burrows |
| author_facet | E. Peters F. Wittrock A. Richter L. M. A. Alvarado V. V. Rozanov J. P. Burrows |
| author_sort | E. Peters |
| collection | DOAJ |
| description | Spectral effects of
liquid water are present in absorption (differential optical absorption
spectroscopy – DOAS) measurements above the ocean and, if insufficiently
removed, may interfere with trace gas absorptions, leading to wrong results.
Currently available literature cross sections of liquid water absorption are
provided in coarser resolution than DOAS applications require, and
vibrational Raman scattering (VRS) is mostly not considered, or is
compensated for using simulated pseudo cross sections from radiative transfer
modeling.
<br><br>
During the ship-based TransBrom campaign across the western Pacific in
October 2009, MAX-DOAS (Multi-AXis differential optical absorption
spectroscopy) measurements of light
penetrating very clear natural waters were performed, achieving average
underwater light paths of up to 50 m. From these measurements, the retrieval
of a correction spectrum (H<sub>2</sub>O<sub>corr</sub>) is presented,
compensating simultaneously for insufficiencies in the liquid water
absorption cross section and broad-banded VRS structures. Small-banded
structures caused by VRS were found to be very efficiently compensated for by
the intensity offset correction included in the DOAS fit. No interference
between the H<sub>2</sub>O<sub>corr</sub> spectrum and phytoplankton absorption
was found.
<br><br>
In the MAX-DOAS tropospheric NO<sub>2</sub> retrieval, this method was able to
compensate entirely for all liquid water effects that decrease the fit
quality, and performed better than using a liquid water cross section in
combination with a simulated VRS spectrum. The decrease in the residual root mean square
(rms) of the DOAS
fit depends on the measurement's contamination with liquid water structures,
and ranges from ≈ 30% for measurements slightly towards the water
surface to several percent in small angles above the horizon. Furthermore,
the H<sub>2</sub>O<sub>corr</sub> spectrum was found to prevent misfits of
NO<sub>2</sub> slant columns, especially for very low NO<sub>2</sub> scenarios, and thus
increases the reliability of the fit. In test fits on OMI satellite data, the
H<sub>2</sub>O<sub>corr</sub> spectrum was found selectively above ocean surfaces,
where it decreases the rms by up to ≈ 11%. |
| first_indexed | 2024-04-12T10:18:31Z |
| format | Article |
| id | doaj.art-c0245680cb684150b48483b7f625d42a |
| institution | Directory Open Access Journal |
| issn | 1867-1381 1867-8548 |
| language | English |
| last_indexed | 2024-04-12T10:18:31Z |
| publishDate | 2014-12-01 |
| publisher | Copernicus Publications |
| record_format | Article |
| series | Atmospheric Measurement Techniques |
| spelling | doaj.art-c0245680cb684150b48483b7f625d42a2022-12-22T03:37:09ZengCopernicus PublicationsAtmospheric Measurement Techniques1867-13811867-85482014-12-017124203422110.5194/amt-7-4203-2014Liquid water absorption and scattering effects in DOAS retrievals over oceansE. Peters0F. Wittrock1A. Richter2L. M. A. Alvarado3V. V. Rozanov4J. P. Burrows5Institute of Environmental Physics (IUP), University of Bremen, Otto-Hahn-Allee 1, 28359 Bremen, GermanyInstitute of Environmental Physics (IUP), University of Bremen, Otto-Hahn-Allee 1, 28359 Bremen, GermanyInstitute of Environmental Physics (IUP), University of Bremen, Otto-Hahn-Allee 1, 28359 Bremen, GermanyInstitute of Environmental Physics (IUP), University of Bremen, Otto-Hahn-Allee 1, 28359 Bremen, GermanyInstitute of Environmental Physics (IUP), University of Bremen, Otto-Hahn-Allee 1, 28359 Bremen, GermanyInstitute of Environmental Physics (IUP), University of Bremen, Otto-Hahn-Allee 1, 28359 Bremen, GermanySpectral effects of liquid water are present in absorption (differential optical absorption spectroscopy – DOAS) measurements above the ocean and, if insufficiently removed, may interfere with trace gas absorptions, leading to wrong results. Currently available literature cross sections of liquid water absorption are provided in coarser resolution than DOAS applications require, and vibrational Raman scattering (VRS) is mostly not considered, or is compensated for using simulated pseudo cross sections from radiative transfer modeling. <br><br> During the ship-based TransBrom campaign across the western Pacific in October 2009, MAX-DOAS (Multi-AXis differential optical absorption spectroscopy) measurements of light penetrating very clear natural waters were performed, achieving average underwater light paths of up to 50 m. From these measurements, the retrieval of a correction spectrum (H<sub>2</sub>O<sub>corr</sub>) is presented, compensating simultaneously for insufficiencies in the liquid water absorption cross section and broad-banded VRS structures. Small-banded structures caused by VRS were found to be very efficiently compensated for by the intensity offset correction included in the DOAS fit. No interference between the H<sub>2</sub>O<sub>corr</sub> spectrum and phytoplankton absorption was found. <br><br> In the MAX-DOAS tropospheric NO<sub>2</sub> retrieval, this method was able to compensate entirely for all liquid water effects that decrease the fit quality, and performed better than using a liquid water cross section in combination with a simulated VRS spectrum. The decrease in the residual root mean square (rms) of the DOAS fit depends on the measurement's contamination with liquid water structures, and ranges from ≈ 30% for measurements slightly towards the water surface to several percent in small angles above the horizon. Furthermore, the H<sub>2</sub>O<sub>corr</sub> spectrum was found to prevent misfits of NO<sub>2</sub> slant columns, especially for very low NO<sub>2</sub> scenarios, and thus increases the reliability of the fit. In test fits on OMI satellite data, the H<sub>2</sub>O<sub>corr</sub> spectrum was found selectively above ocean surfaces, where it decreases the rms by up to ≈ 11%.http://www.atmos-meas-tech.net/7/4203/2014/amt-7-4203-2014.pdf |
| spellingShingle | E. Peters F. Wittrock A. Richter L. M. A. Alvarado V. V. Rozanov J. P. Burrows Liquid water absorption and scattering effects in DOAS retrievals over oceans Atmospheric Measurement Techniques |
| title | Liquid water absorption and scattering effects in DOAS retrievals over oceans |
| title_full | Liquid water absorption and scattering effects in DOAS retrievals over oceans |
| title_fullStr | Liquid water absorption and scattering effects in DOAS retrievals over oceans |
| title_full_unstemmed | Liquid water absorption and scattering effects in DOAS retrievals over oceans |
| title_short | Liquid water absorption and scattering effects in DOAS retrievals over oceans |
| title_sort | liquid water absorption and scattering effects in doas retrievals over oceans |
| url | http://www.atmos-meas-tech.net/7/4203/2014/amt-7-4203-2014.pdf |
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